Charge-coupled devices are broken down to two major categories depending upon the locations of the channel to be formed, including a category of surface-channel charge-coupled devices and a category of buried-channel charge-coupled devices. In terms of the driving scheme, on the other hand, there are charge-coupled devices of single-phase, two-phase, three-phase and four-phase designs. A charge-coupled device of any of these types and designs is essentially a functional device constituted by a MOS structure providing a built-in scanning function and capabilities of storing information. Such a device has its typical applications in image sensing arrays, analog delay devices, and digital filters, as is well known in the art. A charge-coupled device of the buried-channel type has carriers transferred at a certain depth in the silicon substrate, compared to a device of the surface-channel type. Buried-channel charge-coupled devices are generally recognized to excel over surface-channel devices for carrier transfer efficiency and dark current characteristics.
Charge-coupled devices of any of the types and designs presently known however tend to produce large dark currents. The fact is that devices of the buried-channel type have not been improved to such an extent that reduction of the dark current is satisfactory. The attempts which have thus far been made to reduce dark currents in charge-coupled devices include the use of intrinsic gettering for the creation of lattice defects in the silicon substrates and the use of extrinsic gettering by doping with heavy metals. None of these attempts have however proved successful. Where a charge-coupled device is used in an image sensing array, as is frequently the case, the dark current produced by the device impairs the color tones and contrast of the image reproduced and could not produce pictures with acceptable qualities.
It is, accordingly, an important object of the present invention to provide a process of fabricating an improved semiconductor charge-coupled device having a reduced dark current.